1. Field
The field relates generally to an organic light emitting diode (OLED) display. More particularly, it relates to an OLED display that can improve light efficiency and color reproduction rate while reducing the number of black spots.
2. Description of the Related Technology
An organic light emitting diode (OLED) display device is a self emissive display device that displays images using organic light emitting diodes. The organic light emitting diode display differs from a liquid crystal display (LCD) in that it does not require a separate light source, and has relatively small thickness and weight. The organic light emitting diode display exhibits characteristics such as low power consumption, high luminance, and short response time, and has therefore been considered a next generation display device for portable electronic appliances.
OLED devices typically include a hole injection electrode, an organic emission layer, and an electron injection electrode. Holes supplied from the hole injection electrode and electrons supplied from the electron injection electrode are combined within the organic emission layer so as to form excitons. The OLED display device emits light with energy generated when the excitons return to the ground state.
In order to improve the efficiency of the light generated from the organic emission layer, a microcavity effect has been used. In a top light emission device, the microcavity effect uses a theory that light beams are iteratively reflected by a reflection layer (e.g., an anode) and a transflective layer (e.g., a cathode) that have a predetermined gap (e.g., an optical path length) therebetween, and that a strong interference effect occurs between the iteratively reflected light beams such that light beam having a specific wavelength is amplified and light beams having other wavelengths are offset. Accordingly, color reproducibility and luminance are improved in the front of the device.
The microcavity effect provides a color filter effect for transmission of a light beam of a color. The filter effect is dependent on the thickness of the organic emission layer. The thicknesses of red, green, and blue organic emission layers are set corresponding to the respective wavelengths. The thickness of the organic emission layer is set to be the thinnest at the first cavity (thin film cavity) and is set to be thicker at the next cavity (thick film cavity). Since transmission energy is dispersed into several wavelength bands after the second cavity and beyond, an out-coupling effect is highest at the first cavity. Therefore, the light efficiency can be enhanced by using the first cavity structure (i.e., thin film cavity).
Thin film cavity increases the number of black spots depending on the condition of the surface of the anode or of the particles, because the organic emission layer is thin. Furthermore, the thin film cavity is inferior to the thick film cavity in terms of color purity and color reproduction rate. Thus, a blue pixel forming a relatively thinner film cavity than red and green pixels has color purity that is lower than the sRGB standard color coordinate and a color reproduction rate that is lower than that of a typical CRT. Therefore, a method for improving light efficiency while reducing the number of black spots due to the thin film cavity in red and green pixels as well as improving the color reproduction rate of the blue pixel is needed.
The information disclosed in this Background section is only for enhancement of understanding of the background of the described technology and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.